This invention relates generally to optical encoders and, more particularly, to improvements in optical encoders of the type having means coupled to a movable shaft for interrupting light from impinging on a phototransducer in a predetermined fashion as the shaft is moved.
Optical encoders are typically employed to detect the angular position and velocity of a rotating shaft, and can be used, for example, in servos for controlling position and velocity of carriages and print wheels in printers. A code disk, which is typically formed of glass or etched metal, is coupled to the shaft of a motor driving such a carriage or print wheel. The disk contains a plurality of slots arranged in a predetermined circumferential pattern, and it rotates freely within an encoder housing to alternately block and permit the transmission of a light beam onto the phototransducer.
Typically, the code disk includes two annular tracks, one track having a uniform distribution of slots and the other track having a single slot indicative of an index or "home" position. Separate photocells are used for each track, and a sinusoidal signal is produced from one track and a pulse signal is produced for the other.
By monitoring the occurrence of pulses in the pulse signal and the number of cycles occurring in the sinusoidal signal, an accurate determination of the angular position of the motor shaft can be made. Additionally, by detecting the frequency or, alternatively, the peak slope of the sinusoidal signal, the angular velocity of the motor shaft can be determined. These two signals can thus be utilized by servo apparatus to control both the velocity and position of the motor shaft.
When the sinusoidal signal is used in a servo, it is important that its phase angle be accurately ascertainable. This is facilitated by utilizing a pair of complementary photocells adapted to receive light in an alternating fashion and to produce electrical signals of opposing polarities. By adding these two signals, a sinusoidal signal that is symmetric about a precise zero voltage reference can be produced.
Some shaft encoders available prior to the present invention have included means for producing two sinusoidal signals, with one signal leading or lagging the other by 90.degree., depending on the direction of rotation of the shaft. Appropriately incrementing or decrementing an up/down counter can then produce a continuous indication of the angular position of the shaft, regardless of the direction of its rotation.
A major drawback to the shaft encoders of the aforedescribed type arises from the need for a plurality of separate photocells or separately diffused cells on a common substrate. In either case, excessive size and unnecessary complication and expense are necessarily incurred. Additionally, the timing accuracy of the zero-crossings in the sinusoidal signal is susceptible to any imbalance in the sensitivity of the separate photocells, and maintenance of a constant peak voltage level in the sinusoidal signal cannot be easily achieved.
Another drawback to the aforedescribed shaft encoders arises from the use of relatively bulky and costly trimming potentiometers ordinarily used to balance the voltage outputs of the separate photocells. Still another drawback arises from the extreme criticality of the alignment of the code disk. Any slight misalignment of the hub of the disk relative to the array of photocells can result in substantial phase errors in the sinusoidal signal produced by the encoder.
It will be appreciated from the foregoing that there is still a need for an optical shaft encoder that can produce a signal whose phase angle is an accurate representation of the angular position of a rotatable shaft, without involving undue size complexity, or cost and without the need for a precise physical alignment of the code disk. The present invention fulfills this need.